1917 takes place immediately following the German withdrawal from the Somme battlefield to the Hindenburg Line, which took place from mid-February to early April. The German Army had been traumatised by the experience of the Battle of the Somme the previous year, and Field Marshal Paul von Hindenburg and General Erich Ludendorff were now determined to prevent the Allies from building on their gains and forestall another advance by withdrawing to a prepared position. The Hindenburg Line's "battle zone" was over a mile in depth, backed by further reserve positions several miles to the rear, and to attack it would require the Allies to move through miles of devastated countryside, to say nothing of photographing, mapping, and re-registering their guns on a brand new enemy position.

The withdrawal to the Hindenburg Line shortened German lines on the Western Front by 40 kilometres, allowing the Germans to release 14 divisions to form a strategic reserve for attritional fighting. As they withdrew, they scorched the earth behind them: working-age Frenchmen were forcibly evacuated, crops were burned, villages were demolished, bridges were collapsed, and wells were poisoned. The Allies were slow to react: after capturing German orders in February indicating that a retreat was being planned, the French General Louis Frachet d'Esperey argued for a vigorous pursuit. His superior, Robert Nivelle, who had a wider offensive scheme in mind for 1917, demurred. The Allies would make a cautious pursuit, but no attempt was made to press the advantage. Instead, contact was maintained through small-scale patrolling by infantry and light cavalry.

The Germans may have sought to return the ground they had abandoned to France as a wasteland, but this was ground that been behind German lines during the Battle of the Somme the previous year and had survived largely unscathed. There are a number of accounts from Allied soldiers who marvelled at their advance into fresh, green countryside after the blasted hellscape of the Somme: an Australian soldier advancing towards Bapaume remembered that; "after months of solid drudgery in the mud and snow to know that we are soon to leave all this behind for fresh fields and open country without bogs, mires, and shell-holes was great news indeed." Bapaume, a major British objective during the Battle of the Somme but never taken during the battle itself, was liberated by the Australians on 17 March, nine-and-half months after the battle began. While this might be seen as emblematic of the pitiful territorial gains on the Western Front, this was a moment of genuine celebration in the British Expeditionary Force.

The advance into green countryside in pursuit of a retreating enemy was evocative enough that many briefly believed that this was the turning point of the war (with hindsight, this arguably was a turning point, but not for those reasons). These feelings were encapsulated by the British propaganda film The German Retreat and the Battle of Arras, released in the June of 1917, which contrasted the shattered scenes of the Somme battlefield with the green countryside behind it. The retreat could be presented as an Allied victory and provided a much-needed moral uplift to increasingly-war-weary populations in Britain and France.

In summary, the green fields seen in the final assault in 1917 are not unbelievable as this action would have taken place in ground that had largely escaped the fighting in 1916.

Sources:

Gordon Corrigan, Mud, Blood and Poppycock

William Philpott, Bloody Victory: The Sacrifice on the Somme